The present invention relates to a new process for purifying and decolorizing lactose crystals.
Lactose or milk sugar is a standard item of commerce which is generally prepared by crystallizing the same from cheese whey. The basic procedures utilized in crystallizing lactose from whey are outlined in Chapter 12 of Byproducts From Milk, edited by B. H. Webb (1970). Lactose is produced in four general grades of increasing purity, i.e., fermentation, crude, edible and USP. Fermentation grade lactose contains 98% lactose, crude grade lactose, 98.4% lactose, edible grade lactose 99% lactose and USP grade lactose 99.85% lactose. The protein content decreases from 1.0% to 0.01%. The ash and lipid content also decrease. Color decreases from a yellowish color to a white color from the fermentation grade through the USP grade.
The isolation of lactose from whey is generally accomplished by the simple process of concentration and crystallization. The crystals are separated while most of the whey protein and salts are carried off in the mother liquor. However, some of the protein and salts present during crystallization will contaminate the sugar, particularly if they become insoluble during the concentration of the whey. Methods for preparing various grades of lactose generally include methods for keeping the whey protein and ash component solubilized or removing these components prior to crystallization. Even the most effective process for crystallizing lactose will provide a product which has some ash and color.
In the past, preparation of USP grade lactose has required decolorization which has been accomplished by redissolving the crude lactose and treating the same with carbon while boiling. After filtering the carbon, the lactose is again recrystallized. The use of this process in the preparation of edible grade lactose is economically unfeasible though it would be desirable to decolorize the edible grade lactose which tends to have a slight yellowish hue.
It is known that certain amounts of color and impurities can be removed from the lactose by aqueous washing. However, lactose is partially soluble in water and repeated washings cause loss of lactose by solubilization. If the washings are not recycled to the crystallizer, the lactose is lost in sewering the washings.
It is suspected that the yellow color is attributable to the riboflavin contained in the original whey. Riboflavin is known to be soluble with decomposition in dilute alkali (see the definition of riboflavin in the Merck Index, 8th Edition (1968), pages 918-919). The decomposition products are also known to be colored.
It is also known from U.S. Pat. No. 3,816,174 that the treatment of lactose with sodium hydroxide at a high temperature for a short time, or a low temperature for a long time will isomerize the lactose into lactulose syrup. Decolorization of the product is still required which decolorization is accomplished by ion exchange resins.
It has now been found that lactose can be easily and economically decolorized without sustaining substantial loss of the lactose product.